This application claims the priority of Japanese Patent Application No. 165914/1994 filed Jun. 24, 1994 and No. 133773/1994 filed May 23. The above-mentioned hard materials are favored with excellent physical and chemical properties. However, these materials have not been utilized in various fields for practical uses, since fabrication of wide and inexpensive plates or wafers of the materials has not been accomplished to date. Since the hard materials are provided with several physical and chemical advantages, actual applications of the hard materials to various objects are earnestly desired and have been attempted by applying the technology of silicon semiconductor devices to the hard materials. The object has been produce wide plates (or wafers) of the hard materials.
Technologies have already ripened into a definite level capable of producing films of diamond, c-BN (cubic boron nitride) or diamond-like carbon by vapor phase deposition methods. The vapor phase deposition method makes a hard-material of the material by supplying a material gas to a pertinent substrate heated at a suitable temperature, letting the gas react with the hot substrate and depositing a film of the hard material on the substrate in vapor phase. A film of diamond or c-BN is produced by introducing a material gas including hydrogen gas and hydrocarbon gas, or another material gas including hydrogen gas, boron-containing gas and nitrogen-containing gas in the reaction chamber, supplying the material gas on the heated substrate, synthesizing diamond or c-BN by chemical reaction and depositing the synthesized material as a film on the substrate.
There are some methods for exciting the material gas, including, for example, a hot filament CVD method, a microwave plasma CVD method, a radio wave plasma CVD method or a DC plasma jet CVD method. Some methods are capable of making a wide film of hard materials on a substrate. However, the speed of synthesis is so slow that the methods cannot easily make a thick film at present. In accordance with these methods, a long for deposition is needed to make a considerably thick film on the substrate.
Nevertheless, there are still no pure wafers consisting only of a hard material free from a substrate. In other words, at present, there are no diamond wafers or c-BN wafers in their true meanings, because prior technology has not been able to produce a pure diamond wafer or a pure c-BN wafer.
The application of the hard materials, i.e., diamond, c-BN or diamond-like carbon to electronics technology requires wide area wafers of the hard materials. While surface acoustic wave devices on very small diamond substrates have been produced, larger substrates have not.
However the fact that a new device was fabricated on a quite small diamond substrate, e.g., from 5 mm square to 10 mm square, was rather insignificant from an industrial standpoint, even if the device itself exhibited an excellent performance. Since the small substrate allowed only a small number of devices to be made on it, the productivity was poor. The devices made on the small substrate had little practical significance due to the poor productivity.
What brought about the success of silicon semiconductor devices is the ability to treat a wide area Si wafer by the same wafer processes at the same time and make several equivalent devices in a short time. It has been believed that the same would perhaps hold for the hard materials. If diamond, c-BN or diamond-like carbon is to obtain a practical importance as substrates, the material should be formed into wide, round thin plates (wafers). The advent of the wide wafers will enable manufacturers to apply the technology which has been developed by the silicon semiconductor industry to the hard materials.
In the case of silicon, big single crystals with a wide section can easily be grown by Czochralski methods, and mainly 8-inch wafers are produced for making devices at present. 12-inch wafers also can be produced now for silicon.
However, a diamond or c-BN single crystal cannot be grown by the conventional methods, e.g., Czochralski methods, at present. Thus, it is still not promising to produce homogeneous wafers consisting only of a single material of the hard materials, unlike silicon (Si) or gallium arsenide (GaAs). In fact, it is impossible to make wide, homogeneous diamond wafers or c-BN wafers by conventional methods.
This invention gives up the attempt of making a homogeneous wide wafer consisting only of a single material of diamond, c-BN or diamond-like carbon. Instead of starting from the premise of making a homogeneous bulk single crystal, this invention employs a substrate of a different material than the film for making complex wafers containing a non-hard material substrate and hard material films formed on the substrate. The present invention intends to make a hard material film on a commonplace material, e.g., Si, GaAs or so, which can easily be produced or obtained. The complex wafter having a substrate plus a hard material film provides the possibility of making a wide hard material wafer by employing a wide base wafer as a substrate. The base wafer of non-hard material plays the role of the base mount on which the hard material film is deposited. The film on the base wafer is the principal portion of the wafer which will contribute to the production of semiconductor devices or SAWs.
A homogeneous wafer consisting only of pure diamond or c-BN, made by forming a very thick film on the substrate and eliminating the substrate by etching, is still unpractical, because it takes very long time and very much material to deposit such a thick layer. Further, a large inner stress would break the film when the substrate is etched away. Therefore, the production of a freestanding film remains an unpractical object.
This invention is directed to the complex, non-homogeneous wafer having a non-hard material wafer as the substrate of the wafer. The substrate of the wafer does not cause a problem in application, since almost all the devices make use only of the surface of the wafer. This invention employs a non-homogeneous, complex wafer having a Si or GaAs surface on the bottom and a diamond or c-BN surface on the top. The adoption of the complex wafer can overcome the difficulty of making a wide bulk single crystal of the hard materials, because complex wafers can be made by the thin film-formation technology. The term "hard material wafer" in the present invention is quite different from the ordinary wafers produced by slicing an ingot of bulk single crystal. The wafers proposed by the present invention are different from the silicon wafers or the gallium arsenide wafers in their production methods.
This invention satisfies another requirement of the hard material wafers. As mentioned before, it has been attempted to make surface acoustic wave devices on a diamond substrate of 3 mm square or 5 mm square. Such attempts may have a certain significance from the standpoint of technological research. However, such a small substrate is useless for making devices on the industrial scale. The industrial production requires circularity, constant thickness, flatness, unbentness, and large diameter for substrate wafers.
With regard to the area of a wafer, the application to electronics devices demands that the materials have wide circular or rectangular wafers of at least an inch diameter (25 mm). Wafers of a diameter less than an inch cannot be treated by the wafer processes which have been developed by the silicon semiconductor industries. Furthermore, a two-inch diameter wafer is better than a one-inch diameter wafer. The three-inch diameter wafer is more preferable to the two-inch diameter wafer from the standpoint of industrial production, which desires large, flat and smooth wafers.
Fortunately, progress in the technology of vapor phase synthesis enables the production of a considerably broad film of diamond or c-BN on a suitable substrate. However, the blunt possibility of forming wide thin films does not increase the probability of making wide wafers suitable for the wafer process. Wafers having rugged surfaces are useless. Namely, a smooth surface is one of the requisites for wafers. Flatness or unbentness is another matter of significance. The hard material wafer must be a mirror wafer without bending or bow. Here flatness means a long range regularity of a surface. Bending or bow indicates a long range irregularity of a surface. Smoothness is defined as a short range regularity, and ruggedness or raggedness signifies a short range irregularity with may convex or concave imperfections on a surface.
The wafer must be mirror-smooth and flat in order to make devices on the wafer by photolithography. If the wafer is not flat or not mirror-smooth, exact patterns cannot be drawn on the wafer by an optical means.
Films which have been made by the vapor phase deposition have a lot of micro convexes or concaves on their surfaces. Wavy morphology is sometimes formed on the surface of the film. In other cases, granular convexes distribute on the surface of the film. In general, the films which have been made by vapor phase deposition cannot be used as a substrate wafer due to the ruggedness of the surface.
If a wafer suffers from a rugged surface, the wafer could be converted into a mirror wafer by eliminating the ruggedness out of the surface with a polishing apparatus. The polishing would remove the convexes, concaves or wavy morphology out of the surface and would make a flat, smooth wafer. Actually in the case of silicon wafers, mirror wafers are made by slicing an Si ingot into a lot of as-cut wafers, etching the as-cut wafers and polishing the etched wafers by a polishing apparatus. Can the same treatment make flat, smooth hard-material coated wafers? No, the same treatment does not make smooth wafers. The polishing of the hard materials has presented unexpected problems.
Since the convex or concave imperfection would be removed from the surface of the hardmaterial coated wafer by polishing, the starting film must have a sufficient thickness to allow for a margin of polishing. It takes a long time and much material to produce a thick film of the hard materials. This has obvious economic disadvantages.
Another big problem is the difficulty of polishing of the hard materials. The hard-material film can be to difficult to polish. Diamond and c-BN are the hardest materials which are far more difficult to polish than silicon wafers. Diamond or c-BN is polished by diamond powder as an abrasive. The polishing is sometimes called the "together-polishing", because diamond powder shaves diamond by reducing itself during polishing. Lengthy together-polishing can polish surfaces of diamond crystals. The problem of hardness thus can be solved by practicing together-polishing.
However, there is still a more difficult problem. The thermal expansion coefficients are different between the hard material-coated film and the non-hard material substrate. The complex wafer will have a large inner stress due to the difference between the thermal expansion coefficients of the film and substrate. After the complex wafer is produced in a reaction chamber, and is cooled, the complex wafer will bend due to the release of the inner stress when it is removed from of the reaction chamber.
In one case, the complex wafer bends convexly to the side of the film. In another case, the complex wafer bends concavely to the side of the substrate. In yet another case, the wafer stays flat as a whole. The directions of bending are now defined with regard to the side of the film. The wafer bending is classified by the directions of bending. The bending which defines a convex shape on the surface of the film is called "convex-distortion". The bending which defines a concave shape on the surface of the film is called "concave-distortion". The problem of the distortion is still latent in the case of small plates. No distortion occurs for small complex plates of, e.g., 3 mm square or 5 mm square.
The present invention aims at making complex wafers of 1 inch diameter, 2 inch diameter or still bigger diameters. Large amounts of bending appears in the complex wafer due to the difference of the thermal expansion coefficients or the inner stress of the complex film itself. The broadness of the wafer induces a large distortion. The problem of the distortion is quite serious for the hard material coating wafers. In the case of silicon, large, flat mirror wafers can easily be produced since Si wafers are homogeneous without the complexity of a multilayer structure. However, the distortion causes a significant problem due to the non-homogeneity and complexity in the case of hard material-coated wafers.
The distortion of wafers causes many difficulties. The distortion prevents the photolithograpy from transcribing mask patterns exactly on the resist coating of the wafer. This is a big drawback in itself. To begin with, the distortion of a wafer is a large hindrance for polishing the wafer. A conventional polishing machine cannot polish a bending wafer at all. Polishing converts a rugged wafer into a mirror wafer. If polishing is impossible, no mirror wafer can be produced. If a wafer is not mirror-polished, the photolithography is entirely impotent. Thus, it is impossible to make devices on a rugged wafer. Polishing is a fundamental requirement for wafers.
Diamond is the hardest material; there is no harder material than diamond. Thus, diamond is polished by diamond. Namely, diamond plates are polished by an apparatus using diamond powder.
There are two methods for polishing diamond plates. One method adopts free polishing powder. The other adopts fixed polishing powder. Skife polishing which has been known as a way of the former polishing method uses an iron polishing plate and diamond free polishing powder. The method polishes a diamond plate by the steps of gluing the diamond plate to a holder, pushing the holder on a rotary polishing turn-table (round whetstone), supplying polishing liquid containing diamond powder, revolving the turn-table, rotating the holder and whetting the object diamond plate by the physical action of diamond granules. The method has a drawback of producing a large amount of diamond powder waste since it depends on free diamond powder in polishing. The consumption of diamond powder raises the cost of whetting. Another fault of the method is the slow whetting speed and the poor accuracy of finishing.
Another (fixed powder) method grinds diamond plates with a whetstone on which diamond powder is fixed. There are several kinds of diamond whetstones, which are classified by the manner of fixing diamond granules on the whetstones. The whetstone on which diamond powder is fixed by phenol resin, polyimide resin or so is called a resin-bonded whetstone. Another whetstone on which diamond powder is fixed by bronze, cobalt, tungsten, iron, nickel and so forth is called a metal-bonded whetstone. The whetstone on which diamond powder is fixed by plating of nickel etc. is called an electrodeposition whetstone. These whetstones using fixed diamond powder have an advantage of avoiding the waste of diamond granules.
The fixed granule method polishes the surface of diamond plates by the steps of sticking a diamond plate on a holder, pushing the holder on a rotary whetstone table, revolving the whetstone table, rotating also the holder around own axis and polishing the surface of the diamond by the physical interaction with the fixed diamond granules.
All the methods grind diamond plates or diamond films by the physical contact with the diamond granules. The methods depend on the physical action of the diamond powder. Because of the heightened physical interaction, these methods require imposing a heavy load on the polishing faces. The high pressure of the load enables the diamond powder to scuff, scratch, or scrape the surface of the object diamond plates or films. The heavy load also defaces the diamond granules either being fixed on the whetstone or flowing in the liquid. Without the heavy load, the object diamond can be ground no more, slipping in vain on the whetstone.
It has been suggested to develop a polishing method which dispenses with the heavy load. Japanese Patent Laying Open No. 2-26900 (26900/90) proposes a thermochemical method which polishes diamond by chemical reaction by bringing diamond into contact with a heated flat metal table under the oxidizing atmosphere at a high temperature. This chemical method uses no diamond powder.
FIG. 16 and FIG. 17 demonstrate conventional polishing apparatuses for general purposes. The object plate to be polished is hereinafter called a wafer now.
In FIG. 16, a polishing turn table (1) (rotary grindstone) is a diamond whetstone on which diamond powder is fixed by some means. An object wafer (2) is fixed on the bottom of a holder (3). The holder (3) is fixed to a shaft (4). The surface of the holder (3) is parallel with the face of the polishing turn table (1). An air-pressure cylinder or an oil pressure cylinder (5) is mounted on the top of the shaft (4) for pressing the holder (3) via the shaft (4) to the turn table (1). When the apparatus polishes a soft material, the load is unnecessary. But a heavy load is essential for grinding hard materials, i.e., diamond, c-BN or diamond-like carbon. An arm (6) supports the cylinder (5) and the shaft (4). The arm (6) can displace in a radial direction. The region of the whetstone table (1) which is in contact with the object moves in the radial direction for equalizing the defacement of the turn-table (1). The object wafer contacts the central part and the peripheral part of the polishing turn-table (1). The turn-table is worn out uniformly, maintaining the flatness of its face. The flatness of the turn-table ensures the long life time of the polishing table. In the apparatus of FIG. 16, the holder (3) does not rotate. The hard-material film of the wafer is whetted by the interaction between the revolving turn-table and the film.
When a hard-material having a Vickers hardness higher than Hv3000 is polished, the following problems arise.
One problem is the non-uniform polishing which is originated from the imparallelism of the wafer with the polishing table. The wafer must be kept rigorously in parallel with the grinding table in order to polish the whole surface uniformly. However, it is difficult to maintain the wafer in parallel with the polishing table. When the face of the wafer slants, the hard-material film is polished slantingly. Some parts of the film are thinner than other parts, which may be left unpolished. Therefore, the thickness of the film is not uniform in the whole plane. Such an uneven whetting is undesirable. It is important to obtain a uniformly thick film. In the case of a thin film, if the slanting polishing arises, the polishing starts from a corner which comes in contact with the polishing table at first, and then the other parts are later polished. Before some parts are polished at all, the base wafer is revealed under the film. Such a wafer is useless as a hard-material coated wafer. This problem is called a slanting polishing problem.
The other problem is originated when the wafer has an inherent wavy distortion, a concave distortion or a convex distortion. In the case of soft silicon, the convexes or concaves are entirely eliminated where the object is polished by a thickness larger than the heights of the convexes or concaves. However, such an easy solution does not apply to the hard-material coated wafers. The inherent distortion of wafers causes a serious problem in the case of hard-materials. Polishing diamond is far more difficult than silicon. The speed of polishing is far slower. In an initial stage of polishing, the turn-table comes into localized contact with the most prominent part of the film. The area of the contact is small enough, which allows polishing proceed in a pertinent speed. As the polishing operation progresses, the initially-concave parts come into contact with the polishing table. The increase in contact area reduces the pressure per unit area. Thus, the speed of polishing decreases. Eventually, the polishing ceases substantially. An addition to the load is needed to restore the progress of polishing.
However, the apparatus cannot impose an indefinitely heavy load upon the holder. The amount of the load is restricted within a range that will not cause the wafer to break. The limitation of the load causes a shortage of the pressure per unit area, which leaves unpolished parts or insufficiently-polished parts on the wafer. This is a defect of the static polishing apparatus shown in FIG. 16 which does not rotate the holder.
FIG. 17 indicates a perspective view of another prior apparatus which rotates the holder around its axis. The polishing turn-table (1) is a diamond whetstone. A wafer (2) is fixed to the bottom of a holder (3). The holder (3) is fixed to the shaft(4). Bearings sustain the shaft (4) vertical with respect to an arm (6), allowing the shaft (4) to rotate. An oil pressure cylinder or an air pressure cylinder (5) is mounted on the arm (6) above the shaft (4) for applying a load upon the holder (3) via the shaft (4). The arm (6) holds a motor (7) for driving the shaft (4). The rotation torque is transmitted from the motor (7) via a pulley, a belt and a pulley to the shaft (4). The torque rotates the holder (3) and the wafer (2). The wafer is polished by both the revolution of the turn-table (1) and the rotation of the holder (3).
This apparatus positively rotates the shaft (4) by a motor. However, a simpler example can be built by eliminating the motor and holding the shaft rotatably with bearings. Without the positive driving torque, the holder (3) rotates by itself around its own axis. Namely the line velocities of the contact regions are different between the central region and the peripheral region of the turn-table. The difference of the line velocities rotates the holder (3) in a certain direction at a moderate speed. This is a passive rotation. This rotation also is called "accompanying rotation". The wafer (2) rotates on the turn-table whether the motor positively drives the shaft or the speed difference drives the wafer passively.
The rotation of the wafer equalizes the contact of the wafer on the turn-table. In both cases, the rotation of the wafer can solve the problem of the slanting polishing which is caused by an inclination of the shaft with respect to a normal of the turn-table.
However, the rotation of the wafer cannot solve the other problem of the unpolished parts being left on the wavy distorted, convex-distorted or concave-distorted wafer. If the distorted wafer were polished to the bottom of the convexes, a flat wafer could be obtained. However, the amount of polishing cannot be increased so much in the case of hard materials. An application of a heavy load is restricted in order to avoid breakage of the wafer. The limitation on the load compels the apparatus to terminate polishing far before the wafer is completely polished.
The Inventors found that the prior polishing apparatus cannot polish a hard-material coated wafer with a distortion of a height more than 50 .mu.m perfectly, i.e., to the bottom of the convexes. Instead, wide unpolished parts are left on the wafer. Even for wafers of a distortion height between 20 .mu.m and 40 .mu.m, conventional polishing techniques are likely to leave some parts unpolished or imperfectly polished.
For facilitating an understanding of these problems, three kinds of imperfection of polishing are now clarified by referring to FIG. 21, FIG. 22 and FIG. 23. In FIG. 21 to FIG. 23, left figures indicate sectional views of complex wafers having a base wafer (substrate), and a rugged hard-material film, and right figures show the plan views after polishing. FIG. 21 denotes the case of an even wafer. The height of distortion is less than 5 .mu.m. Unpolished parts remain isolated at random within a circle distanced by 5 .mu.m to 10 .mu.m from the circumference. FIG. 22 indicates the case of a concave-distorted wafer. The height of distortion is, for example, 30 .mu.m. A continual, annular part is left unpolished in the middle region within an annular region extending between 5 .mu.m and 10 .mu.m from the circumference. Namely, the periphery and the central part are polished. FIG. 23 denotes the case of a convex-distorted wafer. The height of distortion is, for example, -20 .mu.m. A continual circumference part is left unwhetted. The polishing starts from the central part, develops to the middle region and then pervades to the periphery. This pattern is simpler and more promising than the other two displayed in FIGS. 21 and 22. These are the types of imperfections of polishing. The inclination polishing explained before by FIG. 16 is not shown here, because it appears in the case of a static holder and it is solved by adopting a rotary holder.
The applicability of the conventional wafer process is indispensable for the exploitation of hard-material coated complex wafers, that is, diamond-coated wafers, diamond-like carbon coated wafers or c-BN coated wafers to the fields of electronics, optics or optoelectronics. For example, the technology of photolithography must be able to be applied to the complex wafers for fine processing. Photolithography requires flatness of the object wafers. The various wafer processes have been highly developed in the silicon semiconductor technology. The hard-material coated complex wafers must cope with the conditions presented by the established wafer processes.
The hard-material coated complex wafer must comply with the requirements involved with the fabrication of semiconductor devices. In general, the fabrication technology demands a diameter of more than 2 inch and a thickness of less than 1 mm of a wafer.
The sizes of electronic devices become smaller year by year. The miniaturization of devices requires the reduction of thickness of wafers. The thinning technique has been established for silicon wafers.
Unlike silicon, wide, homogeneous wafers composed of only a single material cannot be produced pursuant to current technology in the case of hard materials, that is, diamond, diamond-like carbon and c-BN, because of the difficulties involved with making big and long single crystals. However, complex wafers can be produced by coating a pertinent substrate with a film of the hard-material. Instead of homogeneous wafers, two-component, complex wafers will be produced for the hard-materials. The nonhomogeneous wafer consists of a substrate and a hard-material film. The hard-materials are hereinafter represented by diamond for brevity.
Diamond-coated wafers can be made by the known plasma CVD method, the hot filament CVD method and so forth. Various materials can be adopted as the substrates. The most convenient substrate is a silicon wafer, because the technology of making and processing silicon wafers has matured. It is easy to obtain flat silicon wafers at low cost.
A complex wafer can be made by depositing a diamond thin film on the substrate by the above-mentioned methods. The surface of the film is rugged. Then the rugged surface of the diamond-coated wafer must be polished into a smooth and flat surface.
However, diamond is the hardest material among all the materials obtainable at present. There is no material harder than diamond. Thus, diamond is mechanically polished by a polishing machine using diamond powder as whetting medium. In the polishing, high pressure must be applied to the surface of the object diamond. Thus, a diamond wafer must endure strong stress. However, a diamond wafer consists of a substrate wafer and a thin diamond film. The mechanical strength of the wafer is determined by the nature of the substrate. High loads affect mainly the substrate in the long run.
Silicon which is used as a substrate is a fragile material. When a silicon wafer is adopted as a substrate, the complex wafer is likely to break, in particular when the wafer has a big diameter and thin thickness. If another material is used as a substrate, the problem will not be solved, since the material is likely to be broken by the high pressure. This problem of high pressure must be solved in order to make a mirror diamond wafer.
There is still another problem. A strong inner stress arises in the complex wafer having a substrate and a film due to the two layered structure. A diamond is synthesized at a high temperature in vapor phase from the excited material gas. Then the wafer is cooled to room temperature. Thermal stress occurs in the complex wafer due to the differences of thermal expansion between the substrate and the film. In addition, a diamond film has inherently intrinsic stress. The thermal stress and the intrinsic stress distort the complex wafer convexly or concavely to a great extent.
Conventional polishing apparatuses whet a flat object by a flat holder and a flat polishing turn-table. The conventional machines are entirely unsuitable for polishing distorted objects. One alternative is polishing a distorted object by gluing a distorted object in a forcibly flattened state on a flat holder, pushing the object by the holder upon the turn-table, rotating the holder and revolving the turn-table in a conventional machine. However, such a superficial improvement would be in vain. One problem is the high probability of breakage of the wafers. Another problem is the difficulty of whetting the film uniformly. Another difficulty is a large fluctuation of the thickness of the film polished. These problems impede the application of the conventional polishing machines to the two-layered wafers having a substrate and a hard-material film.
One purpose of the present invention is to provide a broad complex wafer having a non-hard material substrate and a hard material film.
Another purpose of the invention is to provide an unbent complex wafer without inner stress.
Another purpose of the invention is to provide a smooth complex wafer without micro convexes or micro concaves.
Another purpose is to provide a method of polishing a complex wafer having a fragile substrate and a hard material film.
Another purpose is to provide a method of polishing a complex wafer without breaking or cracking the wafer.
Another purpose is to provide a method of polishing a complex wafer with distortion.
Another purpose is to provide a method of polishing a complex wafer without leaving unpolished parts.
Another purpose is to provide a method of polishing a complex wafer having a substrate and a hard film with little fluctuation of the thickness of the polished film.
Another purpose of the invention is to provide a method of polishing a hard film of a complex wafer with high efficiency.
Another purpose is to provide a machine for polishing a complex wafer having a non-hard material substrate and a hard material film into a mirror wafer.
Another purpose is to provide a machine for polishing a complex wafer without breaking or cracking the wafer.
Another purpose is to provide a machine for polishing a complex wafer with efficiency.